What does Iran say?

He said it was Iran that guarantees the security of the Gulf and the Strait of Hormuz.

Media captionWhy does the Strait of Hormuz matter?

“Unlike the piracy in the Strait of Gibraltar, our action in the Persian Gulf is to uphold international maritime rules,” he said.

Abbasali Kadkhodaei, spokesman of the state watchdog the Guardian Council, said on Twitter that “the law of retaliation is a recognised concept in international law” shortly after the ship’s seizure was announced.

What’s the background to this?

The latest developments come amid a deterioration in relations between Iran and the UK and US.

Tensions between the US and Iran have risen sharply since April, when the US tightened sanctions it had reimposed on Iran after unilaterally withdrawing from a 2015 nuclear deal.

The US blamed Iran for attacks on tankers in the world’s key shipping area since May – Tehran denies all the accusations.

A week later, Iranian boats attempted to impede a British oil tanker in the region before being warned off by a Royal Navy ship, according to the Ministry of Defence. Iran denied any attempted seizure.

International reaction

A White House National Security Council spokesman said the latest incident on Friday was the second time in just over a week the UK had been “the target of escalatory violence” by Iran.

And US Central Command said it was developing a multinational maritime effort in response to the situation.

The US military said it wanted to promote maritime stability, ensure safe passage, and de-escalate tensions in international waters throughout the Arabian Gulf, Strait of Hormuz, the Bab el-Mandeb Strait and the Gulf of Oman.

Given the highly fragile and volatile situation in the Gulf, together with the desperate need to bolster the flagging Iran nuclear deal, was it sensible to detain the vessel carrying Iranian oil off Gibraltar?

How ‘British’ is the tanker?

They must be registered in a country, but that doesn’t have to be the same country as its owners, or have any relation to the cargo, he says. But there must be some link to the UK.

“But how you define UK is relative,” Mr Meade says.

The Stena Impero is Swedish-owned and those on board are Indian, Russian, Latvian and Filipino.

But it’s the UK flag that is important symbolically, he says. “Historically speaking it means that the UK owes protection to the vessel.”

“The UK has political responsibilities to anything that is flagged. And that’s why it’s much more serious than if there just happened to be a British captain on board.”

He emphasises that while it was a political issue, the impact on trade in the region had so far been minimal.

But he warns that if the international community began viewing the Strait of Hormuz as a dangerous place to be, that could create a “very different” scenario.

Highly volatile

The seizing of a British-flagged tanker in Omani waters, empty and inbound to a Saudi port, marks a serious escalation in a whole catalogue of recent incidents in the Gulf.

It comes on the back of the mysterious mining of tankers, the downing of both US and Iranian drones and the near capture of another British-flagged tanker only a few days ago.

Britain wants its response be two things: Measured and multinational.

The government is trying to send a robust message to Iran that this action is unacceptable, not just to the UK but to the rest of the world, but not so robust that it ends up being part of an avoidable US military strike.

This has become a highly volatile situation where not everyone believes in diplomacy. There are figures in Washington who have been pushing for an ever-tougher line with Iran.

And there are figures in Iran, notably in the Revolutionary Guards Corps and the security apparatus, who are quite prepared to push this right up to the brink of a conflict, yet probably stopping just short of one.